1. Field of the Invention
This invention relates to magnetic shielding and in particular to passive magnetic shielding used to contain the fringe magnetic field emanating from a source magnet contained in a magnetic medical treatment instrument.
2. Description of the Related Art
Magnetic medical treatment instruments have long been used in the medical field as a tool in performing non-invasive medical procedures, stereotaxic mapping, and magnetic resonance imaging (MRI). The versatility for providing care with these types of instruments has increased with the advent of more powerful computers to assist the physician in controlling the magnet and processing the data developed by the treatment instrument. However, the fields and gradients created by the source magnets used by these treatment instruments are strong and usually require extensive shielding at many health care facilities.
In the environment of a health care facility, a magnetic field may cause interference with the proper operation of health care monitoring equipment and other electronic health aids. In areas where magnetic medical treatment instruments are used, advisory signs are generally posted to warn about the potential dangers of entering the area. Rooms housing magnetic medical treatment instruments or treatment rooms, have restricted access when the instrument is being used so that patients and other personnel using magnetically sensitive electronic equipment do not inadvertently enter the treatment room are become adversely effected from the operation of the instrument. Since space is at a premium in health care facilities, magnetic medical treatment instruments may be placed in rooms that are adjacent other areas where health care monitoring equipment and other electronic health aids are being used. To limit the areas in the health care facility having restricted access, the treatment room must be properly shielded. Some institutions and agencies impose limits as low as 5 Gauss on the field strength of magnetic fields outside the treatment room.
Conventional shielding systems in treatment rooms at health care facilities are rather elaborate and generally fixed in the structural portions and foundations of the treatment rooms. For example, the walls, the ceiling, and the floor of the treatment room may be lined with iron-based or other magnetic permeable materials in order to deflect and control the magnetic field generated by the source magnet. This shielding is extremely heavy, and may require additional structural support. Thus installation can be difficult, time consuming and expensive.
Often, conventional room shielding must be customized on the site once the magnetic medical equipment is installed to ensure proper attenuation of the magnetic field from the source magnet. These unexpected problems sometimes pose logistical problems for technicians during the initial set-up of the treatment instrument. Highly permeably magnetic materials are generally not easily machined and assembled on the job site. Congruent and matching shapes must be created between adjoining sections of shield to reduce the possibility of fringing the magnetic field.
A primary reason that room magnetic shielding is so heavy and complete is that it is used for MRI machines. These are generally hug cylindrical coils which are kept at a strong current continuously for months and years. Because of the size, the field wall (unless the new, compensating coils are used) is relatively strong. In addition, the size and direction of the field is constant, so that a strongly magnetized shielding, tending in part toward saturation at the wall, will take on a somewhat permanent magnetization of its own, with thermal variations and mechanical vibrations present to realign the magnetic domains more completely. It is observed when MRI machines are turned off that the wall shielding is quite highly magnetized. Unless such shielding is thick and of more expensive very low carbon steel, such a magnetization can be sever, even while maintaining safe levels outside the room, and will be insufficient to maintain safe levels outside after a period of use.
Therefore, in the Magnetic Stereotaxis System, or other strong magnetic field sources which take random directions in a procedure room, it is possible to judiciously design a system with much lighter, thinner, and less encompassing shielding.
Therefore, what is needed is a magnetic shield that guides and shapes the magnetic field emanating from the source magnet of a magnetic medical treatment instrument in such a manner as to attenuate the field in a relatively short distance away from the magnet. The shield would be arranged in close proximity to the magnet to contain the magnetic field around the magnet but still allow proper operation of the magnetic medical treatment instrument. By attenuating the field in a short distance away from the source magnet, rooms adjacent to the treatment room may have unrestricted access. The shield would allow other magnetic sensitive equipment found in rooms adjacent to the treatment room to be operated without interference. The magnetic shield would be standard equipment for a particular medical treatment instrument and obviate the requirement for custom placement and configuration of special shielding in the treatment room. The magnetic shield would be smaller and be of less weight so that the structural requirements of the treatment room may be reduced. The magnetic shield would have a lower cost than conventional methods of shielding the entire ceiling, floor, and walls of the treatment room.
According to the principals of the present invention, a magnetic shield is arranged in close proximity to the source magnet of a magnetic medical treatment instrument and forms an operating space within the volume defined by the shield. The magnetic shield shapes and channels the fringe magnetic field from the source magnetic to allow the magnetic medical treatment instrument to be operated in an environment where other magnetically sensitive electronic equipment may be used.
In one embodiment of the current invention, a magnetic shield is provided around a magnet medical device that generates magnetic fields to shape and channel the field so as to contain the magnetic field in the immediate vicinity of the magnet. Thus, the shield prevents the magnetic field from radiating from the treatment room into surrounding rooms which may have other sensitive electronic equipment that may be disturbed or disrupted by the magnetic field generated by the magnetic medical treatment instrument. The magnetic shield has a ceiling and opposite floor section, and left and right side sections extending between the ceiling and floor sections. The ceiling and floor sections and left and right sections of the shield define the operating space. The left and right side sections are curved members having concave surfaces facing toward each other that give the operating space a generally cylindrical shape. The magnetic reduced device is positional within the operating space.
According to another embodiment of the invention, the shield has the form of a tube with cylindrically shaped sides spaced apart by parallel, planar ceiling and floor sections. The tube has opposite open ends separated by the sides, and a center axis extending between the open ends. The tube has an interior defining an operating space in which the magnetic medical treatment instrument is placed. The operating space has sufficient area to allow a physician to enter and exit the operating space to operate on a patient with the magnetic medical treatment instrument.
According to another embodiment of the invention, a shield is used for controlling the magnetic field emanating from a magnetic medical device. The shield has the form of a circular shaped band having opposite open ends and a center axis extending perpendicularly between the open ends. The band is positioned to surround the magnet at distance from the magnet to allow the physician to enter the interior of the band and operate on the patient. The band has first and opposite, second parallel planar portions. The top surface of the operating table and first and second planar portions are parallel to each other.
In this arrangement the magnetic field emanating from the magnet used in the magnetic medical device may be shaped and contained with the immediate area of the magnet. By guiding the magnetic field in this way, the field may be directed out the open ends of the band into the surrounding room in selected directions. Because magnetic field falls off quickly with distance from the source, the magnetic field channeled out through the open ends of the shield may be dissipated in selected directions without the use of additional shielding in the walls of the treatment room. By providing the shield in close proximity to the source magnet, the magnetic field emanating from a magnetic medical device may be contained within the boundaries of the treatment room so that the operation of the magnetic medical device does not disrupt other electronic and other sensitive medical monitoring and treatment equipment in adjacent rooms. This also allows the construction of smaller treatment rooms. Additionally, by constructing a shield in close proximity to the source magnet, a smaller shield may be provided with a reduced weight and lower cost than room shielding.
In some cases a room is large enough, or a magnet source has small enough projected field, that shields on the ceiling and floor will suffice. In those cases, it is desirable to minimize their sizes and weights. This can optimally be done with appropriate consideration of the relative sizes and distances of these flat shields to that of the source magnet. Usually the floor plate is closer to the source magnet than is the ceiling plate. Therefore its design will need to incorporate greater thickness to reduce saturation, while it may be somewhat smaller in extent because it subtends a larger solid angle at the closer distance to the magnet. In essence, these plate provide regions which do not surround the source magnet, but which instead provide effective return paths for the flux at room boundaries that are closer than the side walls.
With the principle just stated, a finite element analysis (FEA) program can be used to provide specific design by trial and error. In this method, a first trial set of floor and ceiling plates is located in the FEA volume and a worst case magnetic source field applied. The resulting fields in regions beyond the plates and within the plates are calculated. From these the degree of saturation or lack of it in the plates is noted, and sizes and thickness of the plates adjusted accordingly, depending on the material used. A difficulty in this procedure occurs because the source field is so much stronger than the tolerable field outside the shields that sufficient resolution in space and field strength cannot be obtained in practical times with readily available computers. At this juncture in the procedure it is important to assess the field leakage around the plates along with the fields in the plates. If the leakage bulges sharply and significantly at the plate edges, then the plate is not overly saturated, but is too small, at least in one dimension. If the field drops across the plate, but is still too large beyond it, and there are not sharp bulges at the edge, then the plate is too thin, or of a material which saturates too easily, or is of too low permeability.